High speed two wire modem

ABSTRACT

A two wire modem selects a carrier frequency in a baud rate from a predetermined plurality of carrier frequencies and baud rates to communicate with another modem over a communication media in a full duplex mode based on signal and echo characteristics of the communication media estimated by the modem. In addition, the processes of estimating channel characteristics and estimating range are combined in a common start up procedure comprising a plurality of successive time segments for a call modem to communicate with an answer modem over a communication media. Still further, a method is provided for switching from a primary communication media connection to a secondary connection of a data communications network for communication by first and second modems coupled to the network. More specifically, when a failure is detected in the primary connection, the secondary connection is qualified before switching thereto. Similarly, while communicating over the secondary connection, the primary connection is qualified so that communication may be restored thereto at some later time.

BACKGROUND OF THE INVENTION

The present invention relates to modems, in general, and moreparticularly to a modem for selecting a carrier frequency and a baudrate from a predetermined plurality of carrier frequencies and baudrates to communicate with another modem over a communication media of adata communications network in a full duplex mode the selection beingbased on estimated characteristics of the communication media, and amethod of operating a modem by combining the processes of estimatingchannel characteristics and estimating range in a common startupprocedure comprising a plurality of successive time segments.

In a data communication network, digital data among other data, may becommunicated at a data bit rate from one modem to another modem througha communication media, which may be a leased line of the network or adial up connection of a general switched telephone network (GSTN), forexample. Generally, modems operate at a fixed carrier frequency and afixed modulation or baud rate and attempt to optimize the data exchangebit rate based on the conditions of the communication media over whichthey are communicating. In order to accomplish an optimum data bit rate,contemporary modems utilize a startup learning procedure beforecommencing communication during which the modems perform certainpredefined start up procedures which may include a line probingsequence, for example, to establish the media characteristics over whichcommunication will take place. The current state of the art CCITTstandard for two wire full duplex modems is V.32 and V.32bis. An exampleof a modem employing the V.32 standard today includes the Codex Model2264. An example of a state of the art modem using line probing is theCodex Modem Model 3680.

In addition, two wire modems for operating in a full duplex modegenerally employ an echo canceller to cancel from the received signalany near end and far end echoes resulting from its concurrent signaltransmissions. These modems include a ranging sequence as part of thestartup procedure to determine the round trip signal delay time over themedia to and from a remote modem which is used by the echo cancellerthereof. The ranging task is performed separate and distinct from anyline probing tasks. Further, in two wire, full duplex transmissionsystems, there are system nonlinearities which affect not only thesignal transmission, but also both of the near end and far end echoesresulting therefrom. Conventional line probing training sequences do notmeasure the nonlinearities of the echo signals and, for this reason,cannot provide adequate estimates for echo cancellation purposes.

Still further, if during data communication between two modems, amalfunction, like loss of synchronization, is detected, the modemspresently on the market resort to breaking communications and repeatingthe entire startup procedure, including both line probing and rangingtasks, which is a very lengthy and cumbersome retraining process.Moreover, only one of the communicating modems is generally designatedto initiate this retraining process upon malfunction which adds furthercomplications.

The present invention offers aspects intended to alleviate theaforementioned drawbacks of the current modems. These aspects will bebetter understood from a description of the preferred embodiment foundhereinbelow taken together with the accompanying drawings.

SUMMARY OF THE INVENTION

In accordance with the present invention, a two wire modem selects acarrier frequency and a baud rate from a predetermined plurality ofcarrier frequencies and baud rates to communicate with another modemover a communication media in a full duplex mode based on estimatedcharacteristics of the communication media. The two wire modem comprisesmeans for transmitting for a first predetermined time interval a firstline probing signal of varying frequency content over the communicationmedia, means for receiving at least one echo signal of the first lineprobing signal from the communication media in the first predeterminedtime interval, means for receiving a second line probing signal ofvarying frequency content from the communication media in a secondpredetermined time interval, means for estimating signal characteristicsof the communication media based on an analysis of the received secondline probing signal and for estimating echo characteristics of thecommunication media based on an analysis of the received at least oneecho signal of the first line probing signal, and means for selectingthe carrier frequency and baud rate from the predetermined plurality ofcarrier frequencies and baud rates based on the estimates of the signaland echo characteristics of the communication media.

In another aspect of the present invention the processes of estimatingchannel characteristics and estimating range are combined in a commonstart up procedure comprising a plurality of successive time segmentsfor a call modem to communicate with an answer modem over acommunication media. A frequency spectrum of the communication media isconsidered the channel. A round trip delay over the channel between thecall and answer modems is considered the range. The combined procedurecomprises the steps of: estimating a range for one of the call andanswer modems in one time segment of the plurality of successive timesegments of the start up procedure, estimating signal characteristics ofthe channel for the one modem in a second time segment of the pluralitywhile concurrently estimating echo characteristics of the channel forthe other of the call and answer modems, estimating a range for theother modem in a third time segment of the plurality and estimatingsignal characteristic of the channel for the other modem in a fourthtime segment of the plurality while concurrently estimating echocharacteristics of the channel for the one modem.

Another aspect of the present invention involves a method of switchingbetween primary and secondary communication media connections of a datacommunication network. The primary and secondary connections are usedfor communication by first and second modems coupled to the network. Themethod comprises the steps of: communication data between the first andsecond modems over the primary connection, detecting a failure in theprimary connection, and response to the failure detection, qualifying asecondary connection, switching communication of the first and secondmodems from the failed primary to the qualified secondary connection,while communicating data between the first and second modems over thequalified secondary connection, qualifying the primary connection, andrestoring communication of the first and second modems from thesecondary to the qualified primary connection and repeating the firststep.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 are block diagram illustrations of an exemplary datacommunications network model suitable to describe the backgroundenvironment of the present invention.

FIGS. 3A and 3B depict a functionally block diagram schematic of a twowire modem capable of operating in a full duplex mode and suitable forembodying the principles of the present invention.

FIG. 4 is a functional block diagram schematic of a series of modulessuitable for embodying an FFT processor for performing the estimation ofsignal or echo characteristics of the communication media for use in theembodiment of FIGS. 3A and 3B.

FIGS. 5-12 depict "hand-shaking" signal exchange between a modem pairfor start up training and the initiation of retraining during datatransmission in accordance with the principles of the present invention.

FIGS. 13A-13D represents suitable software programming for embodying thefunctional modules of the modem of FIGS. 3A and 3B operating in a callmode.

FIGS. 14A-14D depict suitable software programming for embodying thefunctionally modules of the modem of FIG. 3 operating in an answer mode.

FIG. 15 depicts a method flowchart of a suitable embodiment of anotheraspect of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIGS. 1 and 2 are block diagram illustrations of an exemplary datacommunications network model in which two modems denoted as A and B, ofthe two wire variety are communicating through a general switchedtelephone network (GSTN). In the present example, the modems A and B arecoupled to the GSTN over two wire line connections to hybrids H(A) andH(B), respectively, which convert the two wire connections to four wireconnections of the GSTN. Loop losses of the two wire connections,coupling modems A and B to the GSTN, are represented by the blocks L(A)and L(B), respectively, and the reflected respective impedances arerepresented by Z_(L) (A) and Z_(L) (B). The trunk losses of the networkare lumped according to direction of signal communication and arerepresented by the directional triangles TR.

The illustration of FIG. 1 is used to depict a near-end echo signal 10resulting from the transmitted signal 12 from modem B and reflected fromthe hybrid H(B). The illustration of FIG. 2 depicts a composite signal14 comprising at least two far end echo signals 16 and 18 resulting fromthe transmitted signal 12 from modem B. The far end echo signal 16 is aresidual signal remaining as a result of a non-ideal hybrid matchsituation, which is commonly referred to as "trans-hybrid loss" and isdominant in most far-end echo signals. The other echo signal 18 is aresidual signal remaining as a result of the transmission signal 12traversing the local loop L(A), being reflected off the remote modem Adue to a non-ideal impedance termination in a modem and traversing backalong the same local loop L(A) and then combining with the echo signal16. The echo signal 18 is generally smaller than the echo signal 16since it goes through the loop L(A) twice and the minimum return loss(reflected signal) for a modem is on the order of 11 db in order to meetcertain standards.

In operation, modem B in communicating with modem A transmits its signal12 traversing the local loop L(B), the hybrid H(B) the trunk loss TR,the hybrid H(A) and the loop L(A), which in combination constitute thecommunication media between modems A and B. Concurrently therewith,modem B receives in a full duplex mode not only the transmitted signalfrom modem A but, in addition, the near end echo signal 10 and compositefar end echo signal 14. The communication frequency spectrum of thecommunication media is referred to herein as the channel and the roundtrip delay over the channel between the call and answer modems isreferred to herein as the range. Accordingly, in order to provide aviable estimate of the channel characteristics for optimizing data bitrate and echo cancellation, the near and far end echo signals includingecho nonlinearities should be learned and taken into consideration aspart of the training sequences at the modems A and B.

For the present embodiment, modems A and B may be implemented much thesame or similar to the aforementioned modems marketed by CodexCorporation bearing Model 2264 which have been marketed more than 1 yearprior to the filing date of the instant application and are described inthe publication "2264 Modem Users Manual", 08789, Rev. B, published codeLP, published July, 1989 by Codex Corporation. This manual or manualsare being incorporated by reference herein to provide more specificdetails of the structure and operation of a suitable modem used inconnection of this preferred embodiment.

FIGS. 3A and 3B depict a functional block diagram schematic of a twowire modem capable of operating in a full duplex mode and suitable forembodying the various aspects of the present invention. As in the Codexmodel 2264, the function of the blocks of the embodiment of FIGS. 3A and3B may be implemented in software programs of at least one signalprocessor similar to the type manufactured for Codex Corporation bearingModel No. 60423-51, for example. The modem processor(s) will not bedescribed in detail herein as the use of a signal processor(s) in thecontrol and implementation of modem functions is considered well-known.

Referring to FIGS. 3A and 3B, at the heart of the exemplary modem is afunctional controller module 20 which functions to provide informationto the various other functional modules of the modem in accordance witha predetermined timing sequence which will be described in greaterdetail hereinbelow. The present modem is intended to operate withquadrature amplitude modulation (QAM) for each channel with synchronousline transmission at a selected one of the following plurality ofmodulation or baud rates: 2400, 2743, 2954, and 3200 as will be morefully understood from the description herebelow. The present modem isalso designed to operate at the following data rates: 9600, 12000,14400, 16800, 19200, 21600, 24000, and 25600 bits per second. In thepresent embodiment, the above rates may use a Codex ProprietaryPrecoding Modulation Scheme, combined with 4D trellis coding. Stillfurther, a plurality of carrier frequencies which may be used by thepresent modem include 1600 Hz, 1670 Hz, 1745 Hz, 1828 Hz, and 1920 Hz.The receivers will operate with received frequency offsets of up to + /-7 Hz. A selected carrier frequency and baud rate from their respectivepluralities will be established during a start up procedure after theline has been probed and the operational bandwidth thereof established.Information representing the aforementioned predetermined carrierfrequencies, baud rates and data bit rates are all stored in the memory22 for a selection under control of the controller 20 as will be moreevident from the description found below.

It is understood without having to be shown or described that thepresent exemplary modem includes conventional interchange modem circuitswhich comply with the functionality and operational requirements of theV.24 recommendation of the CCITT and all such interchange circuits areadequately terminated in the corresponding data terminal equipment (DTE)and in the data circuits terminating equipment in accordance withappropriate recommendations for electrical characteristics. In addition,such modems shall accept and pass synchronous or asynchronous data fromand to its corresponding DTE on the appropriate conventional interchangecircuit and under control thereof. The timing, clocks . . . etc., forexample, and data rate selection switching and control are all achievedthrough the conventional interchange circuits.

Referring again to FIGS. 3A and 3B, the modem includes the followingconventional signal generating functional modules: a differential phaseshift keyed generator 24, a tone generator 26, a chirp signal generator28, a train signal generator 30 and a conventionalscrambler/encoder/mapper function 32 which processes the data to betransmitted. A functional switch SW1 selects the output of one of thegenerator modules 24, 26, 28, 30 or 32 to be an input to atransmitter/modulator functional module 34 which in turn generates atransmit signal 36. The signal 36 is conducted through a hybrid circuit38 to the two wire connection 40 to either a leased line or dial-up lineof a telephone network. The generator functions 24, 26, 28, 30 and 32are all selected and enabled by the controller 20 via the signal path42. In addition, the functional switch module SW1 is also controlled bythe controller 20 via the switch control path 44. Still further,information related to the carrier frequency, baud rate and data bitrate along with certain control signals are provided to thetransmitter/modulator 34 from the controller 20 over the data andcontrol path 46.

Received signals are passed from the two line conductor 40 through thehybrid 38 to a combiner function of the modem. A conventional echocanceller function 50 estimates an echo based on perceivedcharacteristics of the channel in accordance information provided to itfrom the controller 20 via path 46. The echo canceller 50 provides theecho estimate to the combiner 48 over path 52 so that the receivedsignal may be relieved of its echo component by the combiner 48. An echoerror is provided back to the echo canceller 50 over path 54 in order toadjust the echo canceller to a more effective level.

The received signal from the combiner 48 is provided over the signalpath 56 to a variety of additional functional modules of the modemincluding a conventional programmable tone detector module 58, areceiver/demodulator/equalizer module 60 and a DPSK receiver 62. Themodem further includes a receiver initialization and control functionalmodule 64 which initializes and controls the module 60 via the signalpath 66. In addition, the tone detect module 58 and initialization andcontrol module 64 are governed by the controller 20 utilizing the path68. The data output of the receiver module 60 is provided to aconventional decoder scrambler module 70 over the data path 72. Themodule 70 processes the data received.

Still further, the modem includes a counter 74 which is used to computethe range MT or NT, as the case may be, which will become more evidentfrom the description found below. The counter 74 may be started by thecontroller 20 using path 76 and stopped by the receiver 60 using path78.

In the present embodiment, a conventional Fast Fourier Transform FFTprocessor 80 operates on the received signals at selected times toestimate channel characteristics and select a communication parametercombination of carrier frequency, baud rate and data bit rate undercontrol of the controller 20 via data path 82. The resulting parametercombination is provided to a decision logic function 84 over the datapath 86. In addition, the selected communication parameter of the remotemodem are received by the DPSK receiver 62 and provided to the decisionlogic module 84 using the path 88. The decision module 84 decides thecarrier frequency, baud rate and data bit rate for use by the modembased on estimated characteristics of the channel over which it iscommunicating with another modem. If the decisional module 84 cannotfind a carrier frequency and baud rate consistent with a desired maximumand minimum bit rate range set by the controller 20, then it generatesan error signal (ERROR). The combined carrier frequency, baud rate anddata bit rate information and ERROR signal are all provided from thedecisional logic module 84 to the controller 20 over the signal path 90for storage in the memory module 22 thereof.

The foregoing described modem may be controlled to initiate a call andthus, be operated in a call mode (hereinafter referred to as a callmodem) or may be controlled to answer a call, and thus, be operated inan answer mode (hereinafter referred to as an answer modem).

An example of operation of the preferred modem embodiment described inconnection with the schematic block diagram of FIGS. 3A and 3B will nowbe supplied in connection with the communication between a call/answertwo wire modem pair which intend to communicate over a communicationmedia such as a lease line or dial-up line of a telephone network likethat described in connection with the network models of FIGS. 1 and 2,supra. The signal flow illustrations of FIGS. 5-12 depict the"handshaking" signal exchange between a call and answer modem pair forstart up training and the initiation of retraining during conventionaldata transmission therebetween. Further, FIGS. 13A-13D representssuitable software programming to embody the functional modules of thecall modem and FIGS. 14A-14D depict flowcharts of software programmingsuitable for embodying functional modules of the answer modem.

To start with, references are made to FIGS. 3A, 3B, 5, 13A and 14A forpurposes of describing a common start up procedure which includes lineprobing the telephone line connection communication media between thecall and answer modems to learn the channel characteristics inestimating the round trip delay from each modem, referred to as ranging.After a call is initiated from a call modem, the answer modem onconnection to the line, shall transmit an answer back tone asrecommended by V.25 and then commence transmitting a pilot tone P1 asshown in FIG. 5 and flowchart block 100. To accomplish this, thecontroller 20 selects and enables the tone generator 36 to generate atone at 400 and 2800 Hz (P1) and controls the switch SW1 to pass thegenerated tone to the transmitter/modulator 34 which transmits the toneoverpath 36 through the hybrid 38 and out over the two wire line 40 tothe call modem. Concurrently, in block 100, the controller 20 of theanswer modem initializes the module 58 for the reception of a tone at1600 Hz (P2). At this time, both modems may be set at symbol timing of2400 Hz.

Simultaneously, the call modem similarly governs the transmission of theP2 tone and initializes its module 58 to receive and detect the P1 toneaccording to block 102. After receiving the answer back tone for atleast 1 second or upon a reception of the P1 tone as detected by themodule 58 in accordance with block 104, the call modem waits for atleast 128 timing periods (at a symbol timing T of 2400 Hz) and thentransmits the phase reversal tone P2 for a period of 16 T by controllingthe tone generator 26 and transmitter/modulator 34 (block 110).Concurrently therewith, the controller 20 of the call modem starts theNT counter 74. Thereafter in the flowchart block 112, the call modeminitial detect module 58 to detect the phase reversed tone P1 andcontrols the transmitter 34 to transmits all zeroes. Then looks for thephase reversal tone P1 in the decisional block 114.

After receiving the P2 tone (block 106), the answer modem initializesits detect module 58 in block 108 to detect the phase reversal tone P2.

The answer modem then looks for the phase reversal tone P2 in thedecisional block 116 and when it receives and detects the tone P2 by themodule 58, it executes the flowchart block 118 which causes thecontroller 20 to wait for at least 64 T, and then control the tonegenerator 26 and transmitter 34 to transmit a phase reversal tone P1 for16 T. Thereafter, in the block 120, the controller 20 of the answermodem initializes the FFT processor 80 to compute the channelcharacteristics estimation resulting from received echo signals.

In the call modem, when the tone P1 is detected as determined byfunctional block 114, the MT counter 74 is stopped by the receivermodule 60 according to the instructions of block 122 and the FFTprocessor 80 thereof is initialized to compute an estimation of channelcharacteristics from a received signal. The resultant digital count ofthe MT counter 74 is representative of the round trip delay or rangebetween the two modems and is stored in the memory 22 by the controller20 for later use in controlling the echo canceller 50.

In the present state, both the call modem and answer modem areinitialized to estimate channel and noise characteristics which isaccomplished by one modem transmitting a known broad band signal, suchas a chirp signal and, at the same time, receiving the echo signaltherefrom, while the other modem at the remote end receives the chirpsignal. Both modems, estimate the channel and noise characteristics fromtheir respective receive signals.

In the present embodiment, according to the flowchart block 124, thecontroller 20 of the answer modem controls the chirp generator 28,switch SW1 and transmitter 34 to transmit a line probing chirp signalwhich is a periodic signal comprised of a series of tones spaced at 37.5Hz apart within a frequency band of approximately 100-3600 Hz. Withinthis frequency band, 3 nulls are transmitted in place of the tones whereone measures the nonlinear distortion, if any, introduced by the channelof the communication media. The line probing chirp signal repeatedlytransmitted at least 64 times for approximately 1.8 seconds by theanswer modem during which time it is receiving echo signals which areconducted to the FFT processor 80 and analyzed for estimating echocharacteristics of the communication media.

Similarly, the instructions of block 126 cause the controller 20 of thecall modem to activate the FFT processor 80 to perform and analyze thereceived line probing chirp signal from the answer modem for estimatingthe signal characteristics of the communication media. Each receivingmodem estimates its respective media characteristics of the broad bandsignal by averaging its spectrum over 64 periods which is the number ofchirp signal periods being generated. Averaging over 64 periods by theFFT processor, provides about 18 db of noise rejection which just aboutcancels out any random noise leaving behind only the known transmittedsignal or echo signal linearly and nonlinearly distorted by the channel.The functional block diagram schematic of FIG. 4 offers a series ofmodules suitable for embodying an FFT processor for performing theestimation of signal or echo characteristics of the communication media.

Referring to FIG. 4, the incoming tones of the line probing chirp signalor echo signal thereof, as the case may be, are gathered in the block130 and saved in a temporary memory buffer according to the functionalblock 132. Since the channel may introduce a frequency offset whichcould make the received signal non-periodic, a frequency offsetcorrection is accomplished, using the frequency offset of block 134 andmixer 136, prior to computing the Fourier transform thereof.

A 256 point FFT processing algorithm 138 is used, in the presentembodiment, for computing the received signal to noise ratio over apredetermined frequency spectrum or a received signal to echo ratio overthe same predetermined frequency spectrum. For this computation, 512time samples or points are gated through the gate 140 at a time asgoverned by the time window signal 142 to the FFT 138. A power signalspectrum is computed for each of the 64 periods over the 1.8 secondinterval by the FFT processor 138 and stored in an accumulator 144. Atiming offset correction TOFF and a frequency offset correction ROT areintroduced to each result out spectrum. The individual spectrums arethen averaged to yield an overall resultant spectrums to either reduceor eliminate random noise. Also in block 144, the resultant averagepower spectrum is squared to yield a squared spectrum which issubtracted from the original spectrum to yield a channel noise spectrum.The output of the block 144 provides both a signal power spectrum and anoise power spectrum to a combiner block 146. The nonlinear distortionintroduced by the channel is also measured in block 148 by averaging theenergy at the null points of the signal spectrum which were introducedby the line probing signal. A block 150 is used to hunt for an optimumcarrier frequency based on the resultant spectrums.

Since the goal of the FFT processor is to maximize the data bit rate fora particular channel according to the estimated characteristics thereof,the number of bits per baud that can be transmitted through the channeland received by the modem receiver for a given BER is calculated. A2-tap DFE model is constructed and based on the channel noise spectrum,the noise at the output of the DFE model, for unit signal is alsocalculated. The DFE model (linear) noise, the nonlinear noise and thesignal level of the various spectrums are weighted and combined in block146 to obtain the total noise at the input of the receivers decoder. Thesignal to noise ratios are established in blocks 152 and 154. In block156, the total noise including distortion above the received and echosignals are computed and scaled to unity. In block 158, the bits/baud isthen computed from a fourier series approximation of the decoders signalto noise ratio. Resulting from block 158 is an optimum carrier frequencybaud rate, and data bit rate within the desired data bit rate rangeprovided from the controller 20. In block 160 the resultant informationis packed in a particular format for providing it to the other modem aswill be fully understood from the description herebelow.

In the decision logic block 84, a final decision algorithm choosesbetween the selected and received parameters of carrier frequency, baudrate and data bit rate in accordance with a predetermined criteria. Thecriteria used by the present embodiment in choosing between the selectedand received carrier frequencies baud rates and data bit rates, is oneof less than or equal to, respectively.

After the answer modem completes the 1.8 second line probing signalgeneration, it completes the instructions of block 124 by disabling thechirp generator 28 and controls the tone generator 26, switch SW1 andtransmitter 34 to commence transmission of the tone P1 for at least 128T. Thereafter, the controller 20 of the answer modem initializes themodule 58 for the detection of the tone P2 and then waits in thedecisional block 164 for the reception and detection thereof. Inaddition, after the call modem completes the channel estimation of block126 for the received line probing signal, the controller 20 in responseto the instructions of block 166 initializes the module 58 for thedetection of tone P1 and waits for the tone P1 to be detected accordingto the decisional block 168. During this time, the call modem iscontinuing transmission of zeroes. Upon detection of the tone P1 by themodule 58, the controller 20 of the call modem terminates the zerotransmissions by controlling the transmitter 34 and generates the toneP2 by controlling the tone generator 26, switch SW1 and transmitter 34.At the same time, the controller 20 of the call modem initializes themodule 58 for the detection of the phase reversal tone P1 according toblock 170. In block 172, the controller 20 of the call modem waits in aloop for the detection of the tone P1 by the module 58.

When the tone P2 is detected by the module 58 of the answer modem, thecontroller 20 thereof responds to the instructions of block 174 andinitiates transmission of the phase reversal tone P1 for 16 T andconcurrently starts the NT counter 74. Moreover, the instructions ofblock 176 cause the controller 20 of the answer modem to initialize themodule 58 for the detection of the tone P2 after 16 T and, then, causethe transmitter to transmit zeroes during which time it waits in adecisional loop according to the block 178 for the reception anddetection of the tone P2.

After P1 is detected by the module 58 of the call modem (block 172),block 180 is executed by the controller 20 thereof which governs thetransmission of the tone P2 for 16 T after waiting for a delay period of64 T. Concurrently, the controller 20 of the call modem initializes itsFFT processor to perform an estimation of channel characteristics basedon received echo signal or signals. The detection of tone P2 by theanswer modem causes the receiver 60 thereof to stop the NT counter 74,and the controller 20 to initialize the FFT processor 80 thereof toperform a channel estimation based on the received line probing chirpsignal. In blocks 184 and 186, the same line probing process isperformed as described supra except that the call modem now istransmitting the line probe chirp signal and performing the estimatedchannel characteristics based on the echo signal thereof and the answermodem is performing the estimation of a channel characteristics based onthe received line probing chirp signal. In the present embodiment, thischannel estimation line probing procedure takes approximately 1.8seconds.

After performing the second line probing task, the selected carrierfrequency, baud rate and data bit rate of each of the call and answermodems are provided to their respective controller 20 which in turnselects and enables the respective DPSK generator 24 to generate thelearned information in packets or frames to the other modem via switchSW1 and transmitter 34. In the block 190, the controller 20 of theanswer modem is directed to initiate the DPSK receiver 62 for thedetection of the transmitted DPSK2 frames from the call modem.

In the present embodiment, a 300 baud DPSK modulation scheme is used toexchange the communication parameter information between the call andanswer modems. A carrier frequency of 1200 Hz is used for DPSKtransmission. The DPSK carrier is stored in memory of the controller 20.The data to be sent is encoded and modulated by governing the generator24 by the controller 20 and sent out over the line 40. Since the abovemethod of modulation (by square wave) may produce a lot of out of bandenergy, the carrier frequency may be stored as a digital prolatespherical wave function, to maximize the energy in a narrower band. TheDPSK receiver 62, in each case, recovers the timing of the signal,demodulates and decodes it and unpacks the information from the remotemodem. The information transferred between the modems consists of thebit rate index for each baud rate, the carrier frequency for each baudrate, the baud rate mask to indicate which baud rate is disabled by thehost, symmetric bit rate flag and symmetric baud rate flag.

After the answer modem detects the carrier of the DPSK2 signal accordingto block 192, the instructions of block 194 are then executed governingthe controller 20 to terminate zero transmission and transmit the DPSK1information while receiving the DPSK2 information. The call modem shallnow look for the DPSK1 carrier from the answer modem and then, afterdetecting two frames of DPSK1 information, shall stop transmitting theDPSK2 data and go to an idle state for approximately 20 T and thenchange its carrier frequency and baud rate to the one recommended by thepreceding line probing procedure. Accordingly, when the answer modemdetects loss of carrier from the call modem it also goes to an idlestate for approximately 20 T and thereafter sets its carrier frequencyand baud rate to that recommended by the decisional logic block 84 ofthe preceding line probing procedure.

Note that according to the above described method in connection withFIG. 5 the processes of estimating channel characteristics andestimating range for the call modem to communicate with the answer modemare combined in a common start up procedure comprising a plurality ofsuccessive time segments. For example, there is a time segmentestimating the range NT of the call modem, another time segment forestimating signal characteristics of a channel for the call modem whileconcurrently estimating echo characteristics of the channel for theanswer modem, another time segment for estimating range NT of the answermodem and yet another time segment for estimating signal characteristicsof the channel for the answer modem all concurrently estimating echocharacteristics of the channel for the call modem. In the presentembodiment, these four time segments are performed successively. Theforegoing described method also provides for a fifth successive timesegment in which the call and answer modems exchange learned informationfor making final decisions.

The next portion of the start up procedure to be described (see FIG. 6)is a training sequence for training the receiver and echo canceler ofeach of the call and answer modems. This training sequence is consideredwell known and outlined in the V.32 and V.32bis specifications and notconsidered in any way a part of the present invention except that thetraining sequences are performed based on the recommended carrierfrequency and baud rate learned from the preceding portion of the startup procedure described in connection with FIG. 5. Thus, the descriptionof this procedure will not require great detail, but rather just a briefoverview. The procedure for the most part is currently being used in themodem 2264 marketed by Codex Corporation. Portions of the procedure arealso described in the U.S. Pat. No. 4,987,569 issued Jan. 22, 1991 andassigned to the same assignee as the present application. In describingthis procedure reference will be made to FIGS. 3, 6, 13B and 14B. Withregard to the functional block diagram embodiment of FIGS. 3A and 3B,the ECQT and CHIRP signals are generated by the chirp generator 28, theTRN signal is generated by the train generator 30 and the rate signalsR1, R2 and R3 are generated by a tone generator as are the tone signalsS and S.

Starting with the answer modem, after the carrier frequency and baudrate have been changed to the preferred numbers, an optional task IR maybe performed to calculate a second or third far end echo of the networkfor use in echo cancellation. In the present embodiment, this task isnot contemplated. The next task is the echo canceller quick train (ECQT)which is performed in accordance with the instructions of block 204 Morespecifically, a special chirp signal is transmitted by the answer modemfor approximately 2 NT and a fast train or instantaneous train of theecho canceller thereof is performed based on the echo of the chirpsignal. In block 206, a tone signal S is transmitted for approximately256 T and then the phase reversal S is transmitted for a 16 T.Immediately thereafter, a special chirp sequence which is used toquickly train the equalizer of block 60 of the receiver is transmittedfor 144 T. Next, in block 208 a signal TRN is transmitted for 2048 Twhich is used by the call modem to further train the equalizer of block60 and the echo signal thereof by the answer modem to further train itsecho canceller. Thereafter, a special rate signal R1 is transmitted andduring this time the answer modem waits to detect an S signal from thecall modem in the decisional block 210. When the S signal is detected,the rate signal R1 transmission is terminated and the answer modemcommences transmitting zeroes according to the instructions of block212. The answer modem then determines whether or not the S signal existslonger than or equal to MT and if so, it initializes itself fordetection of the phase reversal S signal in blocks 214 and 216. Duringthis time the answer modem is continuing to transmit zeroes. Once thesignal S is detected in the decisional block 218, the block 220 is nextexecuted to initialize the answer modem to perform an equalizer fasttrain with the received chirp signal from the call modem for 144 Taccording to a well known DFT algorithm. Thereafter, the answer modemperforms a further equalizer training according to a well known leastmeans square (LMS) algorithm for 2000 T based on the received TRN signalfrom the call modem in block 222.

In the next instruction block 224, the answer modem initializes itselffor the detection of a special rate signal R2 transmitted from the callmodem. Once the signal R2 is detected in block 224, the nextinstructional block 228 is executed and causes the answer modem totransmit first the S tone for 256 T, second the S tone for 16 T, andthen the TRN training signal for 512 T. Thereafter, according to theinstructions of block 230, the answer modem commences transmission of aspecial rate signal R3 which includes the information of a selectedcommunication rate upon which both call and answer modems can agree on,i.e. common to both based on the maximum rate that they can operate at.At the same time, the answer modem initializes itself for the detectionof a signal E1 indicative of the call modem agreeing to the selectedrate. Once the signal E1 is detected in the decisional block 232, theanswer modem terminates the transmission of the rate signal R3 andtransmits the signal E2 for 8 T which is an indication to the call modemthat the selected rate is accepted.

Thereafter, the answer modem transmits a frame of channel coefficientsCC2 for 64 T according to the instructions of block 236 and thentransmits scrambled binary zeroes B1 for 256 T according to theinstructions of block 238, followed by the transmission of a flag F2composed of a predetermined baud/bit pattern. Thereafter, the answermodem is enabled for exchange of data based on the recommended carrierfrequency, baud rate and data bit rate determined in the foregoingdescribed learning process.

Concurrently, the answer modem, after transmitting the E2 sequenceaccording to block 234, initializes itself to detect channelcoefficients CC1 from the call modem in block 242. Once the signal CC1is detected according to the decisional block 244, the answer modeminitializes itself for the detection of a flag F1 in block 245. Once thesignal F1 is detected according to the decisional block 246, the answermodem initializes itself to receive data and receives data based on therecommended rates in blocks 248 and 250.

Now for the call modem. After setting the selected parameters in block202, the call modem waits in a loop according to the decisional block252 to detect an incoming S signal sequence from the answer modem inorder to proceed with the training of its receiver and echo canceller.After detecting the S signal, the call modem initializes itself todetect the S signal in block 254 and waits for reception of the S signalin block 256. After S is detected, the call modem initializes itself forand performs an equalizer fast train utilizing the chirp signaltransmitted by the answer modem for 144 T which is accomplishedaccording to the instructions of block 258. Thereafter, the call modemperforms an equalizer training according to a least mean square (LMS)algorithm for 2,000 T using the received TRN signal from the answermodem using the block 260. Next, in block 262, the call modeminitializes itself for detection of the rate signal R1 and waits in adetection loop at decisional block 264 looking for three consecutiveframes which identically match in order to start transmitting the Ssignal sequence. Upon detection of R1 by block 264, the call modem, inblock 266, transmits an S sequence for a period equal to the round tripdelay measurement MT. The IR task segment may be included in theprocedure at this time. Thereafter, in block 268, the call modem maytransmit the echo canceller conditioning signal for a period of 2 MT inorder to perform a fast train echo canceller task based on the receivedecho signal therefrom. Next, in block 270, the transmitter of the callmodem shall transmit an S sequence for a period of 256 T, followed by aphase reversal sequence for 16 T. The phase reversal serves as a timemarker to train the equalizer. The transmitter next transmits a periodicCHIRP sequence for 144 T as part of the receiver conditioning signal.

In block 272, the call modem transmits the TRN signal segment which is asequence of scrambled binary 1's for a period of 2048 T, and thencommences transmission of a rate signal R2 to indicate the availabledata rates in the call modem. The signal R2 takes into account thepreviously received rate signal R1 and may also take into account thelikely receiver performance for the connection. If the connection isunacceptable, the call modem may transmit a GSTN clear down code. Thetransmission of R2 shall continue until the reception and detection ofthe incoming rate signal R3 from the answer modem. During the R2transmission, the call modem detects the signal S from the answer modemin the decisional block 274 and initializes for the S/ S time marktransition in the block 276 and waits for the time mark detection in thedecisional block 278. Once the transition is detected, block 280 isexecuted to govern the reception of the TRN signal from the answer modemfor use in fine tuning the receiver of the call modem and then the callmodem is initialized for the rate signal R3 detection. The call modemwaits for R3 detection in the decisional block 282 and once detected,the call modem transmits a 16 bit sequence E1 indicating it agrees withthe data rate information provided it by the answer modem in the ratesignal R3.

After transmission of the E1 sequence, the call modem transmits oneframe of channel coefficients CC1 for 64 T in block 286 and thentransmits scrambled binary zero's B1 for 256 T in block 288. Thereafter,in block 290, the call modem is governed to transmit a flag F1 composedof a dibit pattern 11, and then proceed to transmit data at the agreedupon rates of the E sequence. The scrambler and encoder 32 isinitialized at this time. If, however, the rate signal R3 calls for aGSTN clear down, the call modem disconnects from the dial line andeffects a clear down.

Concurrently with the transmission of the CC1 signal, the call modem isinitialized to detect the E2 sequence from the answer modem in block292. Once E2 is detected in the decisional block 294, the call modem isinitialized for the detection of the channel coefficients CC2 from theanswer modem in block 296. When CC2 is detected in the decisional block298, the call modem is initialized for the detection of the flag F2 inblock 300. After the flag F2 is detected in block 302, the call modem isinitialized in blocks 304 and 306 to receive data and receive dataaccording to the preferred parameters previously learned.

During the period of data communication over the communication mediabetween the call and answer modems, a retrain request may be initiatedby either modem for a plurality of reasons. For example, if either modemdetects unsatisfactory signal reception, like loss of equalization, forexample, a retrain shall be initiated to reassess the connection whichmay or may not include the line probing learning process. Another reasonmay be if either of the modems determines that the quality of theconnection is good enough to attempt to increase data bit rate for whichonly a quick retrain may be needed. In the present embodiment, a toneusing the signal states A, B, C, and D is used to initiate retrainingand request the desired start up procedure. The signal states A, B, C,and D are separated by 90 degrees and reside at the respective phases210, 300, 30, and 120 for the present embodiment. In describing theretraining procedures in accordance with the present invention,reference will be made to FIGS. 7-12, 13D and 14D. In the modemembodiment described in connection with FIG. 3, the tone generator 26will generate the tones and the tone detect module 58 shall beprogrammed to receive and detect the tone sequences in accordance withinstructions from controller 20.

FIG. 7 represents a time segmented signal flow between a call and answermodem in which retrain and line probing are initiated and requested,respectively, by the call modem. Initially, both of the call modem andanswer modem are exchanging data according to blocks 310 and 312 and arewaiting in a loop to determine whether or not to request a retrain indecisional blocks 314 and 316. When the call modem detects anunsatisfactory signal reception, it shall transmit the tone in thesequence ADCB in block 318 and wait in a loop in accordance with thedecisional block 320 for the detection of the signal sequence ABCD fromthe answer modem. Concurrently, the answer modem is waiting forreception of the signal sequence ADCB in decisional block 322 and whenthat signal sequence is detected, the answer modem delays for a periodof at least 128 T in block 324 and then transmits the signal sequenceABCD for at least a period of 256 T in block 326.

When the call modem detects the sequence ABCD in the decisional block320, it decides whether or not it desires a line probe retrain in block328. Since this is the case in the present example, the call modemterminates transmission for a period of approximately 20 T and thenchanges its symbol clock T to 2400 Hz to begin executing at the start ofthe program described in connection with FIG. 13A commencing with thetransmission of the tone P2 in block 102. Since the answer modem is notrequesting a line probe retrain, its decision of block 330 causesprogram flow to execute the instructions of block 332 which initiatestransmission of the signal sequence BCDA. During the time the answermodem is transmitting the signal sequence BCDA, it awaits detection ofeither a signal sequence DCBA or the tone P2 in the decisional block 334or the detection of a loss of carrier in the decisional block 336. Inthe present case, the detection of the tone P2 returns the program tothe start of the programming sequence described in 14A with thecommencement of the tone P1 after a predetermined period of silence.Thereafter, the line probing and ranging training tasks are carried outin accordance with the instructions of FIGS. 13A-C and 14A-C asdescribed hereabove.

In the example of FIG. 8, the call modem initiates the retrainprocedure, but the answer modem requests the line probing trainingsequence. In this example, the call modem will execute the blocks 310,314, 318, and 320 as described above and similarly the answer modemshall execute the blocks 312, 322, 324, and 326. But since the answermodem is the modem which is requesting the line probe retrain, it shalltransfer its program execution via the decision block 330 to the startof the program of FIG. 14A wherein it commences transmission of the toneP1 after ceasing transmission for approximately 20 T. The modulationclock T is changed to 2400 Hz for the transmission for the tone P1. Thecall modem awaits the detection of either the tone sequence B, C, D, Aor the tone P1 in the decision block 340 as well as a loss of carrier inthe decision block 342. In the present case, when the tone P1 isdetected, the call modem transfers program execution to the start of thetraining sequence of FIG. 13A in which it commences transmission of thetone P2 in block 102 after a silence period of approximately 20 T. Notethat the modulation clock is changed to 2400 Hz for the transmission ofthe tone signal P2. Thereafter, the call modem and answer modem willproceed through the training sequence of FIGS. 13A and 14A.

In the example of FIG. 9, the call modem is initiating retraining mostlikely as a result of the possibility that the quality of the lineconnection is good enough to sustain a higher data bit rate, but no lineprobing training sequence is requested. Once again, the call modemexecutes the blocks 310, 314, 318, and 320 and the answer modem executesthe blocks 312, 322, 324, and 326. When the call and answer modems reachthe decisional blocks 328 and 330, respectively, neither is requesting aline probe retrain. Therefore, the answer modem transmits the tonesequence B, C, D, A in block 332 and starts the range counter NT.Thereafter, the call modem detects the tone sequence BCDA in block 340and delays for a period of 256 T +/- 2 T in block 344. During the delay,the call modem tests for carrier loss in block 346 and if carrier lossis detected, program execution is transferred to the start of theprogram of FIG. 13A. With no carrier loss and after the delay of 256 T,the call modem transmits the tone sequence D, C, B, A in block 348 andawaits the detection of the tone sequence C, D, A, B from the answermodem in the decisional block 350. In turn, the answer modem detects thetone sequence D, C, B, A in block 334 and stops the range counter tocompute the range NT in block 352. The answer modem next waits for adelay of 256 T +/- 2 T and then transmits the tone sequence C, D, A, B.In decisional block 354, the answer modem awaits detection of the tonesequence D, C, B, A from the call modem. Concurrently, upon thetransmission of the tone sequence D, C, B, A in block 348, the callmodem starts its range counter MT and with the detection of the tonesequence C, D, A, B in block 350, the call modem terminates transmissionof the tone sequence D, C, B, A and computes the range MT in block 356.Thereafter, the program execution of the call modem is transferred tothe start of the quick retrain sequence of FIG. 13B omitting the lineprobing and ranging tasks of FIG. 13A. Simultaneously, the answer modemdetects the tone sequence D, C, B, A in block 354 and terminatestransmission for the tone sequence C, D, A, B in block 355 and returnsprogram execution to the retrain sequence at the start of the program ofFIG. 14B also omitting the line probing and ranging tasks of FIG. 13B.

In the examples of FIGS. 10, 11 and 12, the answer modem is the modemwhich is initiating a retraining sequence. The example of FIG. 10 is onein which the answer modem detects an unsatisfactory signal reception andnot only terminates data communication, but also initiates both retrainand line probing sequences. Referring to FIG. 10, the blocks 312 and 316are executed resulting in the transmission of the tone sequence ABCD inaccordance with the instructions of block 358. The answer modem thenwaits for the reception and detection of the tone sequence ADCB from thecall modem in the decisional block 360. Simultaneously, the call modemis awaiting detection of the tone sequence ABCD in the decisional block362 and when detection is made, a delay of at least 128 T is activatedin block 364 and thereafter, the call modem transmits the tone sequenceADCB in block 366. Upon detection of the tone sequence ADCB in block360, the answer modem, which is not requesting line probe retrain,executes the block 332 next to cause the transmission of the tonesequence BCDA after a delay of approximately 256 T and then waits forthe detection of either the tone sequence DCBA or the tone P2 in thedecisional block 334 or loss of carrier (block 336). Concurrently, sincethe call modem is requesting a line probe retrain, program execution isdiverted from the decisional block 328 to the start of the program inFIG. 13A in which the tone P2 is transmitted after a silence period ofapproximately 20 T. The symbol for modulation clock is changed for thetransmission of P2 to 2400 Hz. When the answer modem detects the tone P2in the decisional block 334, it diverts program execution to the startof the program of FIG. 14A in which the tone P1 is transmitted after asilence period of approximately 20 T at the new symbol or modulationclock of 2400 Hz. Thereafter, both the answer and call modems continueprogram execution as described in connection with FIGS. 13A and 14A.

In the example of FIG. 11, it is the answer modem that is requesting aline probe retrain; therefore, after detecting the tone sequence ADCB inthe decisional block 360, the answer modem diverts the program executionvia block 330 to the start of the program of FIG. 14A as describedhereabove. When the call modem detects the tone P1 in the decisionalblock 340 it diverts program execution to the start of the program ofFIG. 13A after delaying for a period of 64 T. Thereafter, both the calland answer modems operate in accordance with the program execution ofFIGS. 13A and 14A.

In the example of FIG. 12, no line probe retrain is requested by eitherthe call and answer modem. Therefore, the call and answer modems shallgo through the same sequence for computing their respective ranges MTand NT as described hereabove in connection with the example of FIG. 9.Thereafter, program execution for both the call and answer modem willproceed starting at the FIGS. 13B and 14B omitting the trainingsequences of FIGS. 13A and 14A.

Another aspect of the present invention involves the data communicationbetween a call and answer modem over a primary communication mediaconnection, like a leased line of a telephone network, for example,wherein a failure in the connection is detected. Normally, the primaryconnection would be switched to a secondary connection like a dial-upline of a telephone network as part of the GSTN. However, in the case inwhich the carrier frequency, baud rate and data bit rate parameters ofthe communication have been optimized in accordance with the abovedescribed learning process, it is not adequate to select any secondaryconnection, but rather important to prequalify the connection so as tomaintain the optimum parameters in continuing data communication betweenthe call and answer modems.

Therefore, this aspect of the present invention deals with prequalifyinga secondary connection of a predetermined plurality of secondaryconnections with regard to the optimized communication parameters priorto affecting the switch over. Likewise, when operating over thequalified secondary connection, it is desirable to test and qualify theprimary connection in order to restore communication over the primaryconnection at some later time. This method shall be described inconnection with the flowchart of FIG. 15. The process of switching froma primary connection to a secondary connection and restoring the primaryconnection is considered well known and will not be described in detailin the instant application.

Referring to FIG. 15, the method starts in the decisional block 370where it is determined whether or not there is a failure in the primarycommunication media connection over which the call and answer modems arepresently communicating. When a failure is detected, the block 372 isnext considered in which a first of the plurality of secondaryconnections is qualified according to the embodiment described in FIG. 5and flowcharts FIGS. 13A and 14A. If it is determined that the selectedsecondary selection can support communication at the desired combinationof carrier frequency, baud rate and data bit rate, it is accepted by thedecisional block 374; otherwise, the next secondary connection of theplurality is qualified and determined whether or not acceptableaccording to the same criteria in block 376. Once the secondaryconnection is accepted, communication between the call and answer modemsare switched to the secondary connections from the primary connection inblock 378 and thereafter, communication continues over the acceptedsecondary connection. Thereafter, the primary connection is tested andqualified in accordance with the same procedures, i.e. FIG. 5 and FIGS.13A and 14A, in block 380. And if found to support the aforementionedcombination of communication parameters at some later time, the primaryconnection is considered accepted by decisional block 382 and as aresult of the acceptance, communication between the call and answermodems is restored to the primary connection in block 384. And themethod procedure continues back at the decision block 370 as describedhereabove.

It is understood that in connection with this last aspect of the presentinvention, the call and answer modems might may be either 2 or 4 wiremodems, however, in the case of a 4 wire modem there is no need forconsidering channel characteristics with respect to echo transmissions.Therefore, that part of the learning sequence may be omitted.

While the aspects of the present invention have been described inconnection with a preferred embodiment or embodiments, it is understoodthat additions, deletions, and modifications may be made thereto withoutdeviating from Applicants' invention. Accordingly, the invention shouldnot be limited to any specific embodiment, but rather construed inaccordance with the recitation of the appended claims hereto.

We claim:
 1. A two-wire modem for selecting a carrier frequency and abaud rate from a predetermined plurality of carrier frequencies and baudrates to communicate with another modem over a communication media in afull duplex mode based on estimated characteristics of saidcommunication media, said two-wire modem comprising:means fortransmitting for a first predetermined time interval a first lineprobing signal of varying frequency content over said communicationmedia; means for receiving at least one echo signal of said first lineprobing signal from said communication media in said first predeterminedtime interval; means for receiving a second line probing signal ofvarying frequency content from said communication media in a secondpredetermined time interval; means for estimating signal characteristicsof said communication media based on an analysis of said received secondline probing signal and for estimating echo characteristics of saidcommunication media based on an analysis of said received at least oneecho signal of said first line probing signal; and means for selectingsaid carrier frequency and baud rate from the predetermined plurality ofcarrier frequencies and baud rates based on said estimates of the signaland echo characteristics of said communication media.
 2. The two-wiremodem in accordance with claim 1 wherein the estimating means includesmeans for computing a signal-to-noise ratio over a predeterminedfrequency spectrum from said received second line probing signal and anecho-to-noise ratio over the predetermined frequency spectrum from saidreceived at least one echo signal; and wherein the selecting meansincludes means for determining at least one desirable communicationfrequency band of the communication media within the predeterminedfrequency spectrum based on the computed signal-to-noise andecho-to-noise ratios and for selecting the carrier frequency and baudrate based on said at least one desirable communication frequency band.3. The two-wire modem in accordance with claim 2 wherein the computingmeans includes a Fast Fourier Transform (FFT) processor.
 4. The two-wiremodem in accordance with claim 1 wherein the estimating means estimatesthe signal characteristics including non-linear signal distortion of thecommunication media.
 5. The two-wire modem in accordance with claim 1wherein the estimating means estimates the echo characteristicsincluding non-linearities in at least one echo path of the first lineprobing signal in the communication media.
 6. The two-wire modem inaccordance with claim 1 wherein the first and second line probingsignals include a chirp signal ranging from a first frequency to asecond frequency which is repeated a plurality of times over saidrespective first and second predetermined time intervals; and whereinthe estimating means estimates an average of the signal characteristicsbased on an analysis of the repetitive chirp signal of the second lineprobing signal and estimates an average of the echo characteristicsbased on an analysis of the repetitive chirp signal of the echo of thefirst line probing signal.
 7. The two-wire modem in accordance withclaim 1 wherein the selecting means includes means for selecting thecarrier frequency and baud rate based also on at least one desired databit rate.
 8. The two-wire modem in accordance with claim 7 including ameans for generating an error signal if the selecting means is unable toselect a carrier frequency and baud rate in accordance with the desireddata bit rate.
 9. The two-wire modem in accordance with claim 7including means for receiving information from the other modem, saidinformation including a carrier frequency, a baud rate and a data bitrate at which the other modem expects to communicate, and means fordeciding which of the selected and received carrier frequency, baud rateand data bit rate will be ultimately used by the two-wire modem incommunicating with the other modem over the communication media.
 10. Thetwo-wire modem in accordance with claim 9 wherein the deciding meansdecides which of the selected and received carrier frequency, baud rateand data bit rate will be ultimately used for communication based on aless than and equal to comparison criteria.
 11. Method of combining theprocesses of estimating channel characteristics and estimating range ina common start up procedure comprising a plurality of successive timesegments for a call modem to communicate with an answer modem over acommunication media, a frequency spectrum of which being the channel,and a round trip delay over the channel between the call and answermodems being the range, said method comprising the steps of:estimating arange for one of the call and answer modems in one time segment of theplurality of successive time segments of the start up procedure;estimating signal characteristics of the channel for the one modem in asecond time segment of said plurality while concurrently estimating echocharacteristics of the channel for the other of the call and answermodems; estimating a range for the other modem in a third time segmentof said plurality; and estimating signal characteristics of the channelfor the other modem in a fourth time segment of said plurality whileconcurrently estimating echo characteristics of the channel for the onemodem.
 12. The method in accordance with claim 11 wherein the one,second, third and fourth time segments are successive in the start upprocedure.
 13. The method in accordance with claim 11 wherein the onemodem is the call modem and the other modem is the answer modem.
 14. Themethod in accordance with claim 11 including the steps of:selecting acarrier frequency and baud rate for the call modem based on theestimated signal and echo channel characteristics thereof; selecting acarrier frequency and baud rate for the answer modem based on theestimated signal and echo channel characteristics thereof; exchangingthe information of respective carrier frequency and baud rate over thecommunication media between the call and answer modems; and determininga carrier frequency and baud rate for each of the call and answer modemsfrom the selected and exchanged carrier frequencies and baud rates,respectively.
 15. The method in accordance with claim 14 wherein themethod steps thereof are performed in a successive time segment to theone, second, third and fourth time segments of the start up procedure.16. The method in accordance with claim 14 including a successive stepof training a receiver and echo canceller of each of the call and answermodems by exchanging signals therebetween over the communication mediaat the respective determined carrier frequency and baud rate thereof.17. The method in accordance with claim 16 including the stepof:thereafter, exchanging data between the call and answer modems overthe communication channel at the respective determined carrier frequencyand baud rate thereof.
 18. The method in accordance with claim 17including the steps of:during the data exchanging step, requesting aretrain of the call and answer modems by either of the call and answermodems including the steps of estimating the range and channelcharacteristics of each modem.
 19. The method in accordance with claim18 including the step of requesting the steps of estimating the rangeand channel characteristics by the call modem.
 20. The method inaccordance with claim 18 including the step of requesting the steps ofestimating the range and channel characteristics by the answer modem.21. The method in accordance with claim 17 including the steps of:duringthe data exchanging step, requesting a retrain of the call and answermodems by either of the call and answer modems including the steps ofestimating the range for both the call and answer modem without thesteps of estimating channel characteristics.